What a glorious day for boating on the tidal Potomac River around Mason Neck, south of Fort Belvior. A friend asked it we’d like to join him and his family for a day on the water and of course I couldn’t turn down such a generous offer.

We spent most of the afternoon on the river until nearly sunset, a perfect way to end meteorological summer in the Northern Hemisphere. The kids got one final opportunity for outdoor fun, a little swimming and a some tubing to end their summer break from school. Parents got time to relax on Labor Day weekend even if we didn’t get the whole summer off.

I didn’t let the possibility of Bull Sharks ruin my day either, even if Bull Sharks travel into freshwater. As Shark Savers explained,

Bull sharks are unusual because they can adapt readily to fresh water because they can adapt their process of osmogregulation. The kidneys of bull sharks, (and to a lesser extent several other types of sharks) can be gradually adjusted to suit the salinity of the water they are in… This adaptation allows bull sharks to live entirely in estuaries or freshwater.

They also have a fearsome reputation as unpredictable predators that are particularly aggressive towards humans. Still, one shouldn’t have to worry about a Bull Shark swimming from the Atlantic Ocean into the Chesapeake Bay and up the Potomac River, almost all the way to Washington, DC, right?

They’re baaaaack! Not that they were ever gone; they’ve just kept a low profile. Two eight-foot, 220-pound bull sharks were caught in Maryland near Point Lookout, where the Chesapeake Bay and Potomac River meet… it’s important to note that shark attacks are rare; there have been no reports of shark attacks in the Chesapeake Bay despite the fact that at least 12 sharks occur there.

It’s significant to note that while attacks are rare, they are not completely unknown to the area. The Bay Journal referenced an early colonial account: "One letter documented a pretty incontrovertible case of a fatal shark attack on a man swimming in the Potomac" in the 1640’s. One fatality in nearly 400 years? I liked my odds.

That made me wonder though, exactly how far could a Bull Shark travel upstream in freshwater?

Uncountable sources referenced an incident from 1937. Supposedly two commercial fisherman, Herbert Cope and Dudge Collins, caught a Bull Shark near Alton, Illinois on the Mississippi River. That would mean the shark had to swim about 1,150 river miles (1,850 km) upstream from the Mississippi "Head of Passes." A grainy black-and-white photograph of the alleged capture can be seen commonly on the Intertubes although I could not locate a single contemporary record of that encounter. It seemed like it might be plausible, however I remained skeptical absent confirmation.

Even more extreme, there were accounts from 2006 of Bull Sharks caught much farther up the Mississippi watershed, at Lake Pepin in Wisconsin and Minnehaha Creek in Minnesota. This would place the distance at closer to 1,750 miles (2,800 km) upstream through freshwater. None of these accounts came from credible news sources or scientific journals. They all seem to reference one another in an endless loop. Here’s the kicker: the website that seemed to have sparked it all included a disclaimer at the bottom of the page in tiny print, "Any resemblance in the above story to actual fact may be coincidental and could be disregarded, depending on your mood. April Fools!" It was a joke that people have been reporting as fact ever since.

Another rumor, an outright hoax, was debunked in May 2013. Television station WTHI in Terre Haute, Indiana reported No Shark in the Wabash River. "Indiana Department of Natural Resources officials say a report that a bull shark has been found in the Wabash River is just another fish tale."

Given that track record, what should one think of even more remarkable claims from the Amazon watershed? The common refrain is that Bull Sharks "have been found 2,500 miles (4,000 km) up the Amazon River in Perú." That would place the closest location at the Brazil-Columbia-Perú tripoint, the area known as Tres Fronteras in Spanish and Três Fronteiras in Portuguese. A lot of people live there where the Colombian port at Leticia,the Brazilian city of Tabatinga and the Peruvian settlement of Santa Rosa de Yavari all come together, with a combined population of 100,000. One would think there should be at least a sign of news coverage if a Bull Shark had been captured. And yet, I could not find any conclusive evidence, just another round of endless repetition of a remarkable claim.

I never did verify the true freshwater range of Bull Sharks, although I suspect the answer would probably be in the hundreds of miles rather than thousands.